Relationship between surface potential and particle size in soap-free emulsion copolymerization of styrene and methyl methacrylate using a water- or oil-soluble initiator

The soap-free emulsion copolymerization of styrene and methyl methacrylate (MMA) monomers using a water- or oil-soluble initiator was carried out at a constant monomer concentration to synthesize polymeric particles. The relationship between particle size and surface potential was investigated by changing the mixing ratio of MMA to styrene during the copolymerization. Both the particle sizes and zeta potentials were nearly constant when using water-soluble initiator, V-50, because the decomposition of the functional groups of the water-soluble initiator had a large influence on the surface potential. On the other hand, on increasing the molar fraction of MMA monomers, the surface potential of the particles formed during the copolymerization with an oil-soluble initiator, 2,2′-azobis(2-methylpropionitrile), decreased and the particle size increased due to coagulation between particles. This occurred because the surface potential is controlled by the presence of phenyl rings in the styrene monomer. We concluded that both particle size and surface potential can be controlled by adjusting the monomer mixture ratio in the soap-free emulsion copolymerization using an oil-soluble initiator.